This invention relates to a signal processing system for a magnetic flowmeter, and in particular to such a system which automatically maintains the internal accuracy of the system over a wide range of flow rates, and in particular at low flow rates.
In a magnetic flowmeter system, the output of the meter is a voltage proportional to the rate of flow of a fluid flowing through the meter body. This signal is generally of rather low level, and is amplified by a signal processing system to provide an output signal. In presently known magnetic flowmeter systems, the accuracy of the signal processing system, in terms of percentage of flow, decreases dramatically as the flow rate decreases below, say, ten percent of full scale. To compensate for this effect, the signal processing system includes a manual "range" setting which permits the system to operate in a "normal" range for flow rates of between ten percent of full-scale and full-scale, and in a "low" range for flow rates below ten percent. This arrangement has a number of drawbacks. For example, it requires operating personnel to keep watch over the output of the system and make the appropriate change in setting when the flow rate reaches the critical setting. It does not maintain as high a degree of accuracy as desirable in either operating range. It requires calibration of both scales. It requires conversion of the displayed output of the system from one range into the units of the other range (e.g. dividing by ten).